Sodium crosses epithelia in two steps, entry from the luminal side down an electrochemical gradient and exit from the cell by an ATP coupled active step. Using the toad urinary bladder as a model epithelium we have directly measured the entry step using rapid (12 sec) isotope uptake into the tissue. We found that the permeability of the luminal membrane is decreased by maneuvers that increase intracellular sodium or calcium. Further, the effect of increasing intracellular sodium appeared to be mediated by changes in intracellular calcium since removal of external calcium blunted this effect. We also examined the relation between ion transport and cellular energy metabolism. We found that changes in the rate of transport affected the intracellular concentrations of ATP, ADP and inorganic phosphate. The free energy of ATP hydrolysis (Delta-G ATP) was found to be inversely related to the rate of transport. Since the rate of oxygen consumption is regulated by the cytoplasmic Delta-G ATP these results provide an explanation for the effect of sodium transport on cellular respiration.